Grain-drying facilities are conventionally known which combust the rice husk that is one of the biomass fuel in a combustion furnace, supply the generated hot air to a heat exchanger, warm the outside air that has been taken into the heat exchanger, generate the hot air thereby, further add an auxiliary hot-air that has been generated by a kerosene oil burner to this hot air, and supply the mixed air to a grain-drying apparatus. The temperature of the above described hot air is adjusted by mixing the hot air with the outside air, and the hot air is supplied to the grain-drying apparatus as a drying air.